Studies of the provenances, facies, and subsidence histories of Upper Cretaceous and lower Tertiary strata in conjunction with flexural modeling document the tectonic origin and sedimentary evolution of the northern Green River basin in western Wyoming. The area evolved from being part of the Sevier foreland into a nonmarine intermontane basin with the uplift of the Wind River and Gros Ventre Ranges during late Cretaceous time, and by early Tertiary time, subsidence and sedimentation related to these uplifts dominated the northern Green River basin. Sandstone compositions and paleocurrent data indicate an abrupt change in provenance at the beginning of Paleocene time, marking the progressive uplift and erosion of the Wind River and Gros Ventre highlands. Sandstones changed from dominantly sedimentary lithic compositions to dominantly feldspathic compositions when the crystalline core of the Wind River Range was breached. Similarly, paleocurrent trends changed from southeasterly flow directions southward and southwestward, as both the Gros Ventre and Wind River uplifts flooded the basin with detritus. The alluvial sandstone architecture of Upper Cretaceous and lower Tertiary rocks was analyzed in order to document the interaction of allocyclic controls and depositional facies as related to the subsidence history of the basin. This approach proved to be successful only where complicating factors, such as climate and source lithology, could be adequately constrained. Analyses of facies within the lenticular sandstone and shale sequence (Campanian) and the Hoback Formation (Paleocene) suggest deposition during rapid subsidence. The alluvial architecture of Eocene strata (Pass Peak and Wasatch formations) of the Hoback area cannot be easily interpreted in terms of subsidence. Rapid subsidence is indicated for the LaBarge Member of the Wasatch Formation in the Big Piney-LaBarge area. Subsidence analysis for the Hoback, Pinedale anticline, and LaBarge areas documents the patterns and timing of tectonically induced subsidence. A subsidence event occurring at approximately 120 to 115 Ma was probably related to thrusting in the Idaho-Wyoming thrust belt. Another subsidence event at approximately 90 Ma may indicate initial uplift of the Wind River block. The very rapid subsidence event in the Pinedale anticline area during Maastrichtian time is not evident in subsidence curves from the Hoback and LaBarge areas, and thus is probably a manifestation of loading by the Wind River thrust. Rapid subsidence during Paleocene time in the Hoback area is attributed to loading from the Darby thrust and Gros Ventre uplift. Two-dimensional profiling of the northern Green River Basin shows that the basin can be effectively modeled as a flexural depression resulting from extrabasinal and intrabasinal loading on an elastic lithosphere. Two distinct models were used to confirm regional compensation and the flexural response to loading of the lithosphère. Modern basin geometry analysis tested for regional compensation by comparing modeled deflections with observed basin geometry for a given load configuration. Sediment thickness profiling was used to determine the maximum thickness of sedimentary rocks that could have accumulated in the tectonic depression resulting from Darby, Prospect, and Wind River thrusting (assuming instantaneous uplift and adequate sediment supply). Both models are consistent with the concept of basin-margin tectonic loading as the main cause of subsidence in the Green River basin.

North American Paleocene land mammal ages are the Mantuan, Puercan, Torrejonian, Tiffanian, and Clarkforkian. These ages (and associate stages) are subdivided into 16 zones or subzones, varying in duration from 0.1 to 2.9 m.y., defined by widespread species. Although gross evolutionary changes during the first four of these ages are about equal, their durations are very unequal. As defined by magnetostratigraphy and fossil occurrence, the Mantuan is about 0.2 m.y., the Puercan about 1.1 m.y., the Torrejonian about 3.1 m.y., the Tiffanian about 6.1 m.y., and the Paleocene part of the Clarkforkian about 1.3 m.y. in duration. Puercan encompasses normal magnetozone 29, Torrejonian zones 28 and 27, and the Tiffanian–Clarkforkian boundary falls in zone 25. The type Rio Chico Formation of Patagonia is of mid-Tiffanian to Clarkforkian age. Problems in the identification of magnetozones in the San Juan Basin have arisen because an unconformity is present between the Kirtland Shale and the Ojo Alamo Sandstone, and for some years an extra normal chron was falsely identified. When this hiatus is taken into account, marine and terrestrial fossil correlations agree with magnetozone correlations throughout Upper Cretaceous and Paleocene rocks. The Danian stage in marine rocks in North Dakota is equivalent to Mantuan through early Tiffanian; the Thanetian is exactly equivalent to mid-Tiffanian to early Clarkforkian. The rate of Paleocene sedimentation in the major basins of North America does not depart from linearity much more than the contemporary rate of seafloor spreading. Terrestrial rates of sedimentation vary from a peak of 568 bubnoffs (b) (meters per million years) for the Hoback Formation at the Rocky Mountain front to 99 b in the San Juan Basin and 15 b in the Black Peaks Formation in Texas, compared to 2.7 b at Gubbio, Italy. Sedimentation rates along a transect through the Bighorn, Powder River, and Williston basins follow the equation Y = 200X −0.25 , where Y is the sedimentation rate in bubnoffs and X is the radial distance in kilometers from the Absaroka thrust. Absolute taxonomic and morphologic rates of evolution of the most rapidly evolving mammals during the Bugcreekian–Mantuan interval across the Cretaceous–Paleocene boundary peak at 5 genera per m.y. and 3.85 darwins (a rate of measurement defined in the text), the fastest rates known in the fossil record, and decline exponentially to more normal rates of 1 species per m.y. and 0.5 darwins by the Tiffanian. Range charts of 299 species of ungulates, primates, and multituberculates permit ready identification of zones. Seven new species of multituberculates are described, and shape and metrical properties of latest Cretaceous–Paleocene neoplagiaulacid multituberculates are summarized for ease in identification.

Abstract The Wind River Mountains of west-central Wyoming are bounded on the southwest flank by a thrust fault which dips 20° northeast and has a maximum vertical displacement of 35,000 feet. Seismic data show the magnitude and character of the fault zone. The fault originated from an overturned basement fold which was subsequently broken and thrust toward the southwest. Uplift of the mountains began by folding during the Late Cretaceous, continued throughout the Paleocene, and culminated in thrusting at the end of the Paleocene. Nonmarine sediments in the Green River basin adjacent to the uplift were deposited without interruption in a dominantly quiet-water environment, but as uplift progressed, an increasing number of coarse clastics were derived from the mountain flank. After thrusting, early Eocene fluvial sediments from the uplift spread basinward. Gas occurs at Pinedale in tight sandstones of the Paleocene Hoback Formation in a basin facies. Possibilities for both gas and oil exist farther west where cleaner fluvial sandstones interfinger with the basin shales.

Abstract The Green River and Hoback Basins of northwest Wyoming contain very large, regionally pervasive, basin-centered gas accumulations (BCGAs). Published estimates of the amount of in-place gas resources in the Green River Basin range from 91 to 5036 trillion cubic feet (tcf). The Hoback Basin, like the Green River Basin, contains a BCGA in Cretaceous rocks. In this chapter, we make a distinction between regionally pervasive BCGAs and BCGA sweet spots. The Pinedale field, located in the northern part of the Green River Basin, is one of the largest gas fields in America and is a sweet spot in this very large BCGA. By analogy with the Pinedale field, we have also identified a similar BCGA sweet spot in the Hoback Basin. BCGA sweet spots probably always have characteristics in common with conventional accumulations but are different in that they are always contiguous with the underlying more regional BCGA. In this way, they are inseparable from the more regionally pervasive BCGA. We conclude that the probability of forming sweet spots is highly dependent on the presence of faults and/or fractures that have served as conduits for hydrocarbons originating in regional BCGAs. Finally, we propose that the Paleocene “unnamed unit” overlying the Lance Formation be renamed the Wagon Wheel Formation.

The Gros Ventre, southern Teton, northern Hoback, and Snake River ranges and Grayback Ridge of the Wyoming Range, and the buttes of southern Jackson Hole display a remarkably long and nearly complete stratigraphic column. Archean granites and gneisses are strikingly displayed in the eastern fault scarp and high peaks of the Teton Range and the southeast front of the Gros Ventre Range. Paleozoic rocks, representing all periods with the possible exception of the Silurian, aggregate about 6000 feet, and the Triassic, Jurassic, and Cretaceous are represented by more than 12,000 feet of strata. The Paleocene and Eocene are represented by a maximum of about 20,000 feet in the Hoback basin between the Hoback and Gros Ventre ranges, and in the Mt. Leidy highlands north of the Gros Ventre ranges and east of Jackson Hole. The Oligocene is apparently unrepresented except farther north in Jackson Hole, but the Miocene and early Pliocene are represented by the Camp Davis formation several thousand feet thick in Bryan Flats between Hoback Range and Grayback Ridge, in Jackson Hole, and in Star, Grand, and Swan valleys west of the Snake River Range. Volcanic rocks of Tertiary age are intercalated with the sediments in Jackson Hole and Grand and Star valleys, and there are late Tertiary intrusives in the Snake River Range. The Pleistocene is recorded by the Buffalo, Bull Lake, and Pinedale drifts of Kansan, Iowan and Mankato ages. The Gros Ventre and Teton ranges are within the stable platform or foreland area with Paleozoic and early Mesozoic sections of moderate thickness and comparatively simple structures. The faults are largely high-angle reverse faults with pressures from the north and east. The Hoback and Snake River ranges and Grayback Ridge belong to the borders of the southern Idaho geosyncline with a Cambrian (Gros Ventre) to Cretaceous (Frontier) section nearly 90 per cent thicker than the foreland section. Structurally this area is marked by a series of subparallel low-angle thrusts from the south and southwest including the Jackson, Little Granite, Game, Bear, Darby, Absaroka, Ferry Peak, and St. John thrusts. Although the amount of crustal shortening involved in the individual thrusts of this belt is unknown, the effects of this shortening, combined with marked southward and westward thickening of the sedimentary columns, bring about significant differences in succession and thickness in several formations even between points now only a few miles apart. During several field studies at the University of Michigan Rocky Mountain field station, at Camp Davis, Wyoming, detailed stratigraphic sections were measured in the exposed formations in each of the ranges. These sections show numerous persistent thin diagnostic strata not previously noted, and give a clearer picture of the facies changes between the foreland and geosyncline. These variations are shown by graphic sections of the strata of each system. Single measured sections of the Devonian, Mississippian, Pennsylvanian, and Permian successions and of the Bear River and Aspen formations of the Upper Cretaceous are included in the text, and other measured sections have been placed on open file at the office of The Geological Society of America, where they may be consulted.

Abstract Jonah field is located in Sublette County, Wyoming, and lies in the southeastern portion of the Hoback basin, a northwestern extension of the Greater Green River basin. The field is confined by the intersection of two subvertical shear fault zones that form a wedge-shaped structural block. The updip termination at the southwest end of the field is the apex of the block. The downdip limit is somewhat arbitrarily defined as occurring along the synclinal axis separating the basin flank from the Pinedale anticline to the northeast. Within the wedge-shaped block, overpressure conditions exist near the top of the Upper Cretaceous (Maastrichtian) Lance Formation, 2000–3000 ft (610–915 m) above regional occurrence. Immediately to the west and south of the field, overpressure conditions are present near the top of the Upper Cretaceous (Campanian) Mesaverde Group. The trap at Jonah is described as combination structural-stratigraphic. The bounding fault zones form the lateral trap, and the top seal is comprised of the mudstones that are intercalated with the reservoir sandstones of the Lance. Sandstones in the Lance Formation are the principal reservoir at Jonah field. The Lance Formation is comprised of braided to meandering fluvial sandstones intercalated with overbank siltstones and mudstones. Similar sandstone facies in the upper Mesaverde Group are locally productive. The gross thickness of the Lance Formation increases toward the downdip limit of the field. Near the updip termination, the Lance is 2000 ft (610 m) thick, whereas at the northeastern side of the field, it attains a thickness in excess of 3000 ft (915 m). Overpressure increases storage capacity and gas saturation in the reservoir and allows for subtle preservation of better porosity relative to sandstones outside the field boundary. Original gas in place in the Lance Formation is estimated to be more than 8.3 tcf. Subcompartments formed by faults inside the field exhibit better per-well recovery near their updip edge; poorer performance is present in downdip regions of each compartment. Pore pressure in each compartment increases by about 1 psi/ft of depth or more than twice the normal hydrostatic gradient. Pressure data suggest that migration of hydrocarbons into the Jonah field compartment is occurring currently. Liquid condensate yield from the gas production increases with depth. Despite high pressures, production from the deepest sandstones tends to be poor because of low permeability and the impact of hydrocarbon liquids on relative permeability. The lenticular nature of the fluvial sandstones in the Lance has created highly complex reservoir architecture and is a significant challenge to the gas-recovery process. Connectivity is poor, as indicated by the difficulty in correlation of individual sand bodies between wellbores positioned on 40-ac (16-ha) spacing.